In a method for position determination by means of radio waves, so-called measuring in, a radio signal is emitted, preferably within the microwave range, where the signal has good directivity and the property of being reflected from objects, or, alternatively, of being reemitted with a device intended therefor. The reflected signal is received with two antennas, which are arranged so as to be at a distance from each other in a plane substantially perpendicular to the direction to the object. By the distance between the antennas, a wave reflected by the object will have a longer distance of travel to one of the antennas than to the other. This difference in the distance covered gives rise to a phase difference between the received signals. From the phase difference, a reference angle to the object in relation to the antennas may be calculated in a plane which is formed by antennas and object. Such a method is described, for example, in Swedish patent application No. 8403564-1. In this way, each position of the object corresponds to a certain phase difference.
The method is shown geometrically in FIG. 1. The antennas 1 and 2 are placed at a distance d from each other. The object 3, or usually a so-called transponder on this object, the position of which is to be determined, reflects the emitted wave in a direction towards the antennas 1 and 2. Because the antennas are spaced at the distance d from each other, a difference ΔL in the distance covered arises. The difference ΔL gives rise to a phase difference Δφ=φ1−φ2, where φ1 and φ2 are the phase angle for the signal received at the antennas 1 and 2, respectively. From this phase difference Δφ, the geometrical angle θ may be calculated, sin θ∝ΔL∝Δφ.
The angle θ is thus periodically dependent on the phase difference Δφ, as is clear from FIG. 2. This means that there is an interval outside of which the angle θ is no longer unambiguous but may correspond to more than one position. This interval is inversely dependent on the distance d, that is, the interval increases when d decreases. Thus, from this point of view, it is desired to have as small a distance d as possible to achieve a large unambiguous region for the angle θ.
To achieve good directivity in an antenna, it is composed of a plurality of antenna elements to form so-called array antennas. Such an arrangement, of course, gives the antennas a certain physical extent and thus limits the distance d downward. The distance d in FIG. 1 relates, for a pair of array antennas, to the distance between the respective antenna centers.
Hence, the requirement for good directivity conflicts with the requirement for a large unambiguous region. The invention suggests a device for satisfying the requirement for good directivity while at the same time maintaining the requirement for a large unambiguous region.